1. Field of the Invention
This invention relates to a method of separating gas mixture by pressure swing adsorption and an apparatus executing the method and more particularly to technologies suitable for separating oxygen from air and for enriching oxygen efficiently.
2. Description of the Prior Art
In case oxygen is enriched by adsorption and removal of nitrogen from a gas mixture such as air, pressure swing adsorption methods for separating oxygen and nitrogen from air have been used by repeating adsorption and desorption in turn with assistance of pressure changes. Such methods comprise an adsorption process for producing oxygen as product by allowing nitrogen to be selectively adsorbed after feeding air pressurized by a compressor to an adsorber packed with a solid adsorbent such as synthetic zeolite, for example, 5A.3X; a regeneration process for desorbing nitrogen in the adsorbent by depressurizing the adsorber after completion of the adsorption process; and a pressurization process for pressurizing the adsorber with product oxygen or air up to the pressure for adsorption process after completion of the regeneration process. In carrying out pressure swing adsorption methods, because of the necessity of producing product oxygen continuously, it has been a general practice to provide a plurality of adsorbers packed with the above-mentioned adsorbent and change gas flow passages by means of the valves before and after the adsorbers so as to effect the above-mentioned adsorption, regeneration and pressurization processes.
In order to produce product oxygen continuously, it is necessary to provide at least two adsorbers packed with the above-mentioned adsorbent. In operation of a pressure swing adsorption process with a two-column system, for example, the regeneration and pressurization processes are effected in column 100B, while the adsorption process is made in column 100A. Each process of column 100B takes the same time as the corresponding process in column 100A does. Subsequently in column 100A the regeneration and pressuring processes are effected, while an adsorption process is effected in column 100B. It is a basic type of procedure to produce product oxygen continuously by switching over columns 100A and 100B in turn. In addition to a pressure swing adsorption process with two-column system, those with three- and four- column systems are also applied for producing product oxygen continuously. It is important to improve the recovery rate of product oxygen which is expressed as the ratio of product oxygen quantity produced to oxygen quantity in feedstock air.
In the pressure swing adsorption process with two-column system to obtain a predetermined level of oxygen concentration a column in which the adsorption process is occurring is fed continuously with feedstock air so as to continue producing oxygen until immediately before nitrogen adsorption zone reaches the product outlet end of adsorption bed. In this case, in the vicinity of the product-outlet end there exists the adsorbent which can still adsorb more nitrogen, i.e., there exists oxygen which can still be produced as product by further adsorption of nitrogen from feedstock air. The remaining oxygen is discharged outside the system without being taken out as product at the stage of shifting from the adsorption process to the succeeding regeneration process by means of vacuum pump etc., thus reducing the above-mentioned recovery rate of product oxygen.
As a measure to prevent lowering in recovery rate of product oxygen in a two-column system, there is disclosed in Japanese Laid-Open Patent Application No. SHO. 53-140281, a method for improving the loading efficiency of the adsorber and hence, the recovery rate of product oxygen by controlling longitudinal width of the nitrogen adsorption zone, paying attention to the relation between the longitudinal width of the nitrogen adsorption zone and adsorption pressure conditions in the adsorber in the course of the adsorption process. According to the above-mentioned Japanese Patent Application, a single column system is employed to achieve improvement in recovery rate of product oxygen by depressurizing the adsorber, to discharge product oxygen at reduced pressure, in accordance with adsorption pressure conditions so as to allow longitudinal width of the nitrogen adsorption zone to be reduced for the purpose of reducing remaining oxygen quantity in the nitrogen adsorption zone at the outlet end of packed bed which is discharging product oxygen, before the nitrogen adsorption zone reaches the outlet end.
Further, in Japanese Laid-Open Patent Applications No. SHO. 59-199503 and No. SHO. 56-45724, are described methods for improving the recovery rate of product oxygen by introducing a pressure equalization step wherein high-enriched oxygen from column 100A is transferred upon completion of the adsorption process in column 100A and completion of the regeneration process in column 100B until pressures of columns 100A and 100B become equal.
Furthermore, it is reportedly mentioned that improvement in the recovery rate of product oxygen can be achieved by carrying out the method described in Japanese Laid-Open Patent Application No. SHO. 50-155475. This method using column 200A and 200B, as shown in FIG. 6, comprises steps 1a, 1b, 1c and 1d which will be mentioned below.
At step 1a, while column 200A is under adsorption process, column 200B, which has already completed the adsorption process, is in the course of regeneration process. Secondly, at the stage of shifting from step 1a to step 1b, supply of feedstock air into column 200A is ceased, and regeneration process in column 200B is completed. At step 1b, oxygen which is highly enriched in column 200A is supplied into column 200B via nozzle part. The supply is continued until the pressure in column 200A becomes equal to or slightly higher than that in column 200B. At the time of shifting to step 1c after completion of step 1b, the nozzle at the top of the column 200A is connected to the nozzle at the bottom of column 200B, and the enriched oxygen remaining in the vicinity of the top of column 200A is transferred into column 200B via the nozzle at the bottom thereof. The high-enriched oxygen supplied from the nozzle at the top of column 200A at step 1B is eventually discharged from the nozzle at the top of column 200B by the gas transfer. Further, after shifting from step 1c to step 1d, nitrogen with high concentration is fed through the nozzle at the bottom of column 200A. With this high-concentrated nitrogen, oxygen remaining in column 200A is purged out and transferred into column 200B. After completion of the above-mentioned series of steps, column 200A is subjected to regeneration process and column 200B which is fed with feedstock air is subjected to adsorption process. This method aims at improving the recovery rate of product oxygen by purging out the oxygen remaining in column 200A with high-concentrated nitrogen to recover the oxygen.
Out of the above-mentioned conventional technologies, according to the technology described in Japanese Laid-Open Patent Application No. SHO. 53-140281, supply of feedstock air is ceased before break-through of the nitrogen adsorption zone at product outlet end of adsorption bed, and the longitudinal width of the nitrogen adsorption zone is controlled to become narrower by reducing the pressure in the adsorption column to the level not more than half of the adsorption column. There is, however, limitation in narrowing the longitudinal width of the nitrogen adsorption zone so as to recover the oxygen in the vicinity of the product outlet end of the adsorption bed, in case the concentration of product oxygen from the outlet is maintained at a predetermined value. Furthermore, according to this technology, there remains the adsorbent which does not reach break-through due to depressurization of the adsorption column and discharging of the gas in the column before the break-through. Such an adsorbent does not contribute to separation of feedstock air so that it can not be used effectively. Therefore, this technology poses a problem in that oxygen recoverable as product remains more or less in the vicinity of the outlet end of the adsorption bed, and no attention is paid to the reduction in the effect on improving the recovery rate of product oxygen, and also to the reduction in the quantity of the oxygen which can be produced per unit volume of the adsorbent due to the presence of the adsorbent which does not contribute to the separation of feedstock air.
On the other hand, in the conventional technology which introduces the pressure equalization step as described in Japanese Laid-Open Patent Applications No. SHO. 59-196503 step at a pressure P.sub.1, while column 300B is subjected to regeneration step at P.sub.2, as shown in FIG. 7A. At the time of completion of these steps, supply of feedstock air to column 300A is ceased and oxygen which is enriched in column 300A is fed from the nozzle at the top of column 300A into column 300B via nozzle at the top of column 300B, as shown in FIG. 7b. The supply of oxygen into column 300B is made until the pressure of columns 300A and 300B become (P.sub.1 +P.sub.2)/2, i.e., equal to each other. After completion of the pressure equalization step, column 300A is shifted to the regeneration step, while column 300B is subjected to pressurization step using product oxygen. In the pressurization step, it is required to cease the adsorption step when column 300A is in the adsorption step under a condition that a considerable amount of high-enriched oxygen remains in the adsorbent. (There is a case that more than half of the adsorbent bed is filled with the high-enriched-oxygen.) This is because as soon as high-enriched-oxygen is transferred into column 300B and the latter is subjected to the adsorption step, the high-enriched oxygen needs to be discharged. Once the nitrogen adsorption zone reaches breakthrough at the outlet end of the adsorption bed in column 300A, less-concentrated oxygen is introduced into column 300A, during the pressure equalization step and discharged from column 300B during the adsorption step. Thus, the adsorption step of column 300A needs to be completed under a condition that the high-enriched-oxygen is held in the vicinity of the outlet end of the adsorption bed. Therefore, the pressure swing adsorption process including the pressure equalization step poses a problem in that no consideration is taken on lowering of effect on improving the recovery rate of product oxygen due to oxygen recoverable as product oxygen remaining more or less in the outlet end of the adsorption bed. Since much high-enriched oxygen remains in the adsorption column during the adsorption step, the nitrogen adsorption zone can proceed only up to the medium height of the column 300A. In such a state, a great deal of adsorbent which can adsorb more nitrogen remains in the adsorbent bed, so that adsorbent can not be utilized efficiently. Therefore, there is another problem in that no consideration is taken on the reduction in the quantity of oxygen which can be produced per unit volume of adsorbent.
Further, in the prior art described in Japanese Laid-Open Patent Application No. SHO. 50-155475, it is envisaged that greater improvement in the recovery rate of product oxygen can be achieved as compared with the above-mentioned other prior arts, because the oxygen remaining at the outlet end of the adsorption bed is purged out with nitrogen having a high concentration and recovered as product as mentioned above.
In the above-mentioned conventional technologies, however, a common problem, which no has not been taken into consideration, resides in that each of the above-mentioned steps is effected, after ceasing supply of feedstock air. Supply of air and suspension thereof require starting and stopping of the compressor etc., resulting in discrete operations which may cause operational problems. To solve this point at issue, the pressure swing adsorption process with two-column/two-series system, which involves in practice operation of four columns, is applied to achieve continuous operation. In the two-column process, since feedstock air supply is stopped without fail, there is a problem in that no consideration is taken to improvement in operationability in continuous operation.
Still further, there is a common defect in that no consideration is taken to the fact that a great quantity of high-enriched oxygen remains in the adsorber in the course of adsorption step. Besides, there is a common problem in that no attention is paid to the reduction in the quantity of oxygen which can be produced as product per unit volume of the adsorbent, since when nitrogen adsorption zone reaches half way in the adsorption column, supply of feedstock air is ceased so that the adsorbent which does not contribute to the separation of feedstock air remains more or less in the adsorbent bed. Furthermore, there is another common problem in that suspension of feedstock air supply causes a reduction in the quantity of feedstock air to be treated per unit volume of the adsorbent, if the same operational time is allocated for each of the above-mentioned processing, and that reduction in the quantity of oxygen which can be produced as product take places. This indicates that the quantity of the adsorbent required for obtaining the same quantity of oxygen for the same cycle and for the same oxygen concentration will increase. Accordingly, since the adsorbent is expensive, increase in plant costs such as the cost of adsorbent at the time of plant construction and also the cost of adsorbent for replenishment during ordinary operation is inevitable.